U.S. Pat. No. 6,594,011 issued Jul. 15, 2003 describes an apparatus which employs a substrate with an array of capture elements immobilized on the top surface of the substrate. When the array is exposed to a fluid sample, patterns of binding events occur in the array depending on the analytes present in the sample. The patterns of binding events are captured by directing a beam of polarized light through a prism abutting the bottom surface of the substrate. The beam is directed at an angle to obtain total internal reflection (TIR) and to generate an evanescent field in the plane of the array. The pattern of binding events is imposed on a detection device such as a CCD camera and compared with a stored image of the array captured prior to exposure to the sample. The localized changes in the intensities of reflected light and the locations of those changes in the array are representative of the various analytes present in the sample.